Apple has parental controls specifically designed for your MacOS devices. With the parental controls you can monitor and limit the time your child spends online. Be sure you have an account created for each new internet user in order to place restrictions on some family members and not all.
To turn on and edit parental controls for an existing user account:
Choose the Apple Menu (upper left corner) and select System Preferences
Click Parental controls and unlock the lock icon by clicking on it. You may be prompted to enter your admin name and password, or touch ID.
Select the user you want to edit restrictions for. You can enable or disable access to the Internet, apps, and the iTunes store. You can set time limits by day or bedtime and block apps and services from accessing your child's data. If you have multiple children using the device, you can copy parental settings from one user to another to set restrictions on multiple children.